Defroster circuit



Patented June 24, 1952 UNITED' STATES @FF l 2,601,466

DEFROSTER CIRCUIT Benjamin l`)".,"1"l'1`omas, Cincinnati, Ohio, assigo'V td ANBO; Mllilfaicfllrng Crpblati; Cihil; nati, Ohio',- a" corporation of Delaware pplitiJilxieZo, 1950Selial N0. 169,195

2 Claims. 1l`

This invention relates to defrosting` refrigerator evaporators and is particularly appli cable to defrosting the evaporators ofelectrically' operated refrigerators of theY household type. Various methods and means have heretofore been provided to eliminate the coating'of frost that accumulates on the evaporator of such refrigf erators during extended operation thereof,1par'' ticularly Where ambient conditionsl of high` humidity exist. In some areas this' high hu-v midity condition exists for a' considerable period of time and when it is accompanied byhighambient temperatures the problem of the accumulation of frost is a severe one.' This is accentuated in cases where a large" primary evaporator is relied upon for producin'gfood preservation temperatures in the foodcompartment and low freezing temperatures in` the freezing compartment.

Previous attempts Jto-solve this'probiem have only been partially successful since they have required costly modification of standard refrigeration systems and controls or'have notbeencapable of functioning with sufficient rapidity to melt the surface coating of -ice or froston the evaporator without likewise incitingv or unduly raising the temperature of frozen' food stored in or on the evaporator.-

The present inventionovercomes these" diniculties and permits the defrosting of a'refrigerator evaporator bythe rapid and uniformap'- plication of heat derived from an Velectric resistance element disposed upon or'adiacent the areas of an evaporator where the frostaccum-4 ulation is normally the most severe.V

The invention contemplatesA the provision of heating means which is readily separable'from the evaporator surfaces from which" frost accumulation is sought to be removed,` in order to render such heating means accessible for cleaning, repair or replacement. 1

An object of the invention is the provision of an electric circuit and control therefor which will insure the periodic defrosting of a" refrigerator evaporator automatically.

Another object of the invention isfto" provide" a defrosting device wholly independent of and separate from the hermetically sealed systems employed to providerefrigeration. y

Another object' of the `invention o provide an electrically actiiateddefro'stingm ansi'n'chjunction with an operating and control circuit which producesk the desired result with certainty and rapidityH and at the saine time in an extremely simple and inexpensive manner.

A further Objectis to provide an electric circuit containing a series ofsafety devices which` operate consecutively order to insure againstY A still further object is` to provide 4a controlv for a defroster circuit which avoids the use of relays within the electrical system and of shutoff valves within the refrigeration system.

The novel features that I consider characteristic of my invention are set forth in the appended claims. both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of a specic embodiment when read in conjunction with the accompanying drawing, in which Fig. 1 illustrates a circuit according to the invention as applied to thev refrigerant evaporator of an electrically operated refrigerating apparatus and Fig. 2 shows an enlarged, fragmentary section taken along line 2--2 of Fig. 1.

The invention is applicable to refrigeration systems of various types but in the illustrative embodiment it is shown as applied to a system in which arefrigerant compressor preferably sealed in a dome supplies a suitable refrigerant gas such as dichlorodiiluromethane to a condenser Wherein it is liquefied and supplied through'suitable flow control means toan'evap--A matioally in the drawing.

In the apparatus of the present invention power is provided for thel motor of the com-- pressor through powerlines mand Il of a volt A. C. line.` In circuit with `the power line Il and starting relayv'Md ofthe' rrngtor-coinl pressor l5 is a timror"clock-operated'switch 'i2 The invention itself, however,

with their connecting lines, constitute' the principal instrumentalities of such a4 system and are shown more or less diagramand a conventional temperature control I3 having movable contact G and xed contact H. Control I3 is actuated by a conventional bulb I3a mounted in heat exchange relationship with evaporator 20. A rise of temperature in evaporator 20 increases the pressure in bulb I3a and this pressure rise is transmitted through tube I3b to a Sylphon bellows in I3, the movement of which causes movable element G to make contact at H. This control is well known in the art and will not be described in detail.

The line II is connected to the movable element A of timer I2 and fixed element C thereof is connected to the movable element G of thermostat I3. The other side of the starting relay is connected through conduit I 4A to line III and to a heating element consisting of a straight continuous length 29 and a sinuous length 29a of tubular element having a high resistance, secured to the evaporator 2i). A similar heating element 38, 32a is connected through line I'Ia to xed contact E of a temperature responsive device IS through a safety device I'I such as a thermostatic control device responsive to excessive heat or current or both, disposed in heat exchange relationship with evaporator 20 and the heaters.

The temperature sensitive device I6 comprises a temperature sensitive switch having a control bulb I5@ disposed adjacent the evaporator. The movable element D of the switch is connected to the fixed element B of the timer I2. The other fixed element F of switch It is connected to the fixed element C of timer I2. The heaters 29-30 are preferably tubular, constructed of nickel chromium alloy wire sealed in a heat conductive tube but insulated therefrom with a refractory material such as magnesium oxide.

The heaters are advantageously formed so that they lie between or adjacent the refrigerant-containing passagesV 22, 24 in evaporator 2B. The

construction shown may be varied somewhat and the heaters may be mounted otherwise than as shown and mayvhave courses extended over the side and back of the evaporator 2i) depending upon the defrosting requirements of the evaporator design. It is desirable that the heaters be secured to theV primary refrigerating surfaces, i. e., those in which the evaporator passages are contained or to which they are secured.

In order to supply heat to the back plate 5S by conduction, the heater may conveniently be attached to the evaporator by extensions 55a integral with or secured to plate 5S so that they clamp against course 29 of the heater (see Fig. 2). Similar means 55, 5l, 58, 59 and E@ are provided for holding the heaters with respect to the end plates 52 and E3 and the bottom plate 5I and conducting heat thereto. Clamping means 6I and 62 are provided for holding the front edges of the heaters to the top plate 50 and bottom plate 5I.

As will be noted, the evaporator 20 is closed at the sides and back, and it is also closed at the iront by an access door (not shown) provided with suitable gaskets for sealing the interior so that it is not subject to the air currents which pass through the food compartment and over the external surfaces of evaporator 2). It is important to prevent the accumulation of frost on the interior surface of evaporator 2t.

In the illustrative embodiment, the refrigerant is supplied from compressor I5, and it successively passes through conduit 2Ia, condenser I8, flow control device 2I, and circuitous passage 22 in the bottom plate of theevaporator., From the end of this passage the refrigerant iiows through a connecting passage to another circuitous passage 24 at the top of the evaporator at the end of which is provided a header 26 from which refrigerant gas is removed through the suction tube 2'1 and drawn into the suction side of the compressor at 21a.

The timer I2 is a clock-operated mechanism which by the use of mercury or snap acting switches permits the rapid making and breaking of electrical circuits carrying a considerable amount of current. Its setting is adjustable and is preferably set to make contact at A and B every twenty-four hours, at a time when the refrigerator will not likely be in use.

In operation the clock-operated switch is set so the circuit to the compressor is made from line II through movable contact A and fixed contact C, and normally closed temperature control contacts G and H. When the circuit is in this condition the compressor cycles normally through the temperature responsive element I3 which, as above indicated, is responsive to temperatures of the evaporator or 'the food storage compartment, for instance, through bulb I3a. When the temperature has risen to a degree determined by the setting of the thermostat I3, movable contact G moves away from fixed contact H and opens the motor-compressor circuit thus described.

The timer is set to'permit periodic defrosting, as above indicated. When so set, the time clock starts the defrosting operation by breaking the contacts A and C and making contacts A and B. This stops the compressor, if the motor circuit is not already broken at G-H, and applies power to the heaters 29, 29a, 36, 30a, through circuit II, A, B, movable contact D of the defrost thermostat I 6, i-lxed contact E of the defrost thermostat I5, Ila, I'i, 32,3611, 29a, 29 and I0. Element 28 is simply a jumper for conveniently connecting the heaters in series.

The temperature responsive device I 6 is so adjusted and related to the evaporator 2D and the heaters 29 and 3Q that it will break the contacts D and E and make contacts D and F when the temperature has risen to a point at which adequate defrosting has been completed. The output of the heaters is designed so that this occurs in a relatively short time, say ve to ten minutes, and in any event a time insufficient to deleteriouslyr aiect the frozen foodstuffs or ice cubes within the evaporator. The making of contacts D and F reapplies power to the compressor through a by-pass circuit as follows: II, A, B, D, F, C, G, H, I4, I4a and Ill.

It is important to set the time clock to break contacts A and B and reestablish A and C a short time period after the time when defrost thermostat I6 is calculated to have established connection between contacts D and F. This provides a safety feature which Yinsures against the possibility that the defrost thermostat I6 might not operate. Accordingly, even though the latter does not open the heater circuit at D and E. the circuit is broken at A and B, and the compressor circuit reestablished through A and C.

A still further protective device is provided by a second thermostat I1 which is sensitive to an elevation of temperature in an area closely adjacent the evaporator 20 and heaters 29 and'30. Device I'I may also be made sensitive to excessive current. In either case device I1V is arranged so that, if it feelsl a4 predeterminedv elevation Vin temperaturer oru an excessive currentA ilow, for instance by reason of the failure of the heater thermostat I6 and the timer I 2, it effectively breaks the heater circuit. For convenience of disclosure, thermostat Il is shown adjacent and alongside of evaporator 20. In actual practice, however, it is desirable to place this thermostat in the upper portion of the refrigerator cabinet and it may be depressed in a cavity within the insulation, and closely adjacent the inner liner thereof.

It is obvious that the application of heat to the evaporator for more than the relatively short time required to defrost it would not be advantageous both from the point of view of possible spoilage of frozen foods stored therein and of possible damage to the refrigeration system by supplying heat thereto in an amount more than that which it is designed to accommodate. The controls provided by the present invention and their relationship to the operation of the system are adequate to insure against these contingencies and provide a series of safety devices operating consecutively, in effect, a triple safety device. These controls, however, are extremely simple in their construction and assembly, and the diniculties and risks involved in the use of valves in the refrigeration system and relays in the electrical or control system are avoided as well as the cost thereof.

The precise adjustment of timer I2 and thermostats I6 and I1 will depend of course on a number of factors including the size and capacity of the evaporator and the amount of frost calculated to have accumulated thereon during the previous period during which refrigeration has occurred. I have found, however, that a defrosting system may be constructed in accordance with the principles of the invention by using the following equipment under the conditions noted:

Compressor I5: 1A; H. P. of the reciprocating type.

Evaporator 20: Horizontal frozen food typel constructed of 13" x 22 refrigerated top and bottom plates joined into a box section with 8" aluminum back and side panels, the whole comprising approximately 11/2 cu. ft. of volume and 6 1/2 sq. ft. external surface area.

Condenser I8: Tube and nn construction of such design as to give efticient operation with the above evaporator and compressor at zero degrees to 10 degrees F. at normal control setting,

Cabinet size: 9 cu. ft. of refrigerated space. Cabinet construction conventional including outer shell and inner liner, breaker strip, door outer shell, door inner liner, insulation approximately 3%" of glass or rock wool.

Defrosting heaters, 29, 30: Tubular heaters of conventional resistance wire surrounded by refactory oxides such as magnesium oxide in turn surrounded by a nickel plated copper sheath having a diameter of approximately 1A".

Heater for top evaporator plate: 390 watts, length approximately 105 inches.

Heater for bottom evaporator plate: 500 watts, length approximately 100 inches.

Defrost thermostat I6: Vapor filled construction, single pole, double throw operation, high temperature contacts made at approximately 50; low temperature contacts made at approximately F.

Safety thermostat I1: Conventional bimetallic single pole, single throw construction mounted at top of cabinet liner and opening at temperature indicative of over-heating operation (80- 90 F.)

Time switch I2: Self-starting synchronous motor operating cams to actuate a single pole, double throw snap switch every twenty-four hours maintaining one set of contacts approximately twenty minutes and the other set for the remainder of the twenty-four hour period.

Having thus described my invention, I claim:

1. In an electrical refrigerator of the type energized by two supply mains and comprising a compressor motor and an evaporator, a defrosting device comprising, in combination: a connection from one supply main to said motor, an electric heater disposed adjacent said evaporator, a connection from said one supply main to said heater, a timing device, a rst single-pole, double-throw circuit selector having a movable contact connected to the other supply line, said first selector being actuated by said timer to stop the motor by circuit breaking and t0 energize the heater by circuit making when defrosting is initiated, said selector including` a first xed contact in circuit with said motor and a second fixed contact, a thermostat disposed adjacent to said evaporator, a second single-pole, doublethrow circuit selector having a movable contact connected to said second lfixed contact, said second selector being actuated by said thermostat to de-energize the heater by circuit breaking to restart said motor by circuit making when defrosting is completed, said second selector including a third xed contact in circuit with said heater and a fourth xed contact in a conductive path with said iirst xed contact, and means coupling the first circuit selector to said timer for reactuation by said timer to assure restarting of said motor by circuit making and de-energizing of the heater by circuit breaking at the end of a predetermined interval.

2. In an electic refrigerator of the type comprising an evaporator, a heater element and a compressor motor for circulating refrigerant, the combination of clock-actuated means having two positions foralternatively energizing the motor or conditioning the heater for energization, a thermostat, thermostat-actuated means, operable when the heater is conditioned for energization, for alternatively assuring the energizing of said motor or the energizing of said heater, said thermostat-actuated means comprising a selector, said selector comprising a movable contact actuated by the thermostat and two spaced fixed contacts, a motor energizing circuit for said compressor motor which is closed when said movable contact engages one of said xed contacts, and a heater energizing circuit which is closed when saidmovable contact engages the other of said xed contacts.

BENJAMIN D. THOMAS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,913,433 Doble, Jr. June 13, 1933 2,081,479 Fink May 25, 1937 2,244,892 Newton June l0, 1941 2,313,390 Newton Mar. 9, 1943 2,400,168 Roach May I4, 1945 2,495,378 McCabe Jan. 24, 1950 2,522,199 Shreve Sept. 12, 1950 2,551,163 Rickert et al. May 1. 1951 

